不同培養條件及醣類前處理對藍綠菌Thermosynechococcus sp. CL-1之固碳及單醣類產出之影響

Abstract

Due to the global warming caused by the increasing CO2 concentration in atmosphere in recent decades, several carbon capture and sequestration (CCS) technologies are under researched. Biological carbon mitigation (BCM) is considered as a sustainable and potential process which uses autotrophic organisms such as microalgae and cyanobacteria to absorb CO2 from atmosphere through photosynthesis. Then large amounts of produced biomass can be used as biofuel which can reduce the usage of the gasoline. The residue of biofuel extraction can be also used as the feedstock for combustion. The whole recycle process achieves zero waste. In this study, the combination of the advantage of the chemical-alkaline-absorption and BCM is applied as the technology of carbon fixation from one of the main CO2 emission sources, power plant. CO2 has much more solubility in the alkaline solution and becomes HCO3- or CO32- as the carbon source for cyanobacteria. For meeting this practical requirement, thermophilic and basophilic cyanobacteria Thermosynechococcus sp. CL-1 (TCL-1) was chosen in this study. In order to increase the biomass productivity and carbon fixation rate, the higher surface-area-ratio flat panel was used as the photobioreactor (PBR) with high initial biomass concentration. After finding the best initial biomass concentration, the 23 factorial experimental design was used to study the macronutrient and micronutrient ratio of the MF medium for higher growth and carbon fixation rate. From several researches, cyanobacteria have the ability of accumulation high carbohydrate content as the feedstock for fermentation to bioethanol in certain conditions. Nitrogen deprivation seems as the condition that can accumulate the carbohydrate in several researches. Therefore, this condition was also researched in the optimized medium in this study. Furthermore, selection of the most suitable pretreatment method is crucial in achieving a high level of bioethanol production. Therefore, the pretreatment for carbohydrate of TCL-1 was studied and the content and composition of carbohydrate and monosaccharides under nitrogen deprivation were analyzed. The results show that the maximum biomass productivity 155.6 ± 7.8 mg/h/L and CO2 fixation rate 240.7 ± 7.8 mg CO2/L/h are reached in the condition of 3 g/L initial biomass concentration without pH control and modified MF medium of 5-fold macronutrient and 3-fold micronutrient ratio. The nitrogen deprivation in this modified medium can achieve the highest biomass productivity 124.1 ± 2.7 mg/L/h, CO2 fixation rate 201.2 ± 2.7 mg CO2/L/h, and biomass increment 34.9 ± 3.0% at the 7.29 mM initial DIN concentration. In carbohydrate analysis, one-step hydrolysis (4% H2SO4) followed by enzyme amyloglucosidase for TCL-1 carbohydrate pretreatment was established by two-way ANOVA. By using this pretreatment, it can be found that glucose (89.3-91.9%), xylose (6.4-7.3%) and arabinose (1.8-3.5%) are the main monosaccharides of TCL-1 in the cultivation condition of this study. The maximum carbohydrate content 55.3 ± 0.47%, maximum glucose content 50.5 ± 0.47%, and maximum carbohydrate productivity 53.5 ± 12.7 mg/L/h are reached in the 0.29 mM initial DIN condition. Moreover, from SEM, TCL-1 is 23% longer in the sufficient DIN concentration (29.17 mM) than that in deficient DIN concentration (0.29 mM). This is equivalent to the biomass increment. Therefore, TCL-1 may not have cell division. However, cell divisions were observed in both conditions. If cell division occurred, Monod equation was used as kinetic regression. If cell division didn’t occur, the biomass growth curves and carbohydrate productivity of TCL-1 were developed by empirical kinetic models, Logistic regression, and Modified Logistic regression, respectively, the carbohydrate accumulation curves by the Luedeking-Piret equation, and the DIN consumption curves by Modified Luedeking-Piret equation in the nitrogen deprivation conditions (0.29-7.29 mM) for the future application. The results for both kinetic studies show that all those equations fit well with experimental data and the cell divisions are confirmed.